Optical Fiber and FBG Basics
From fiber structure to Bragg reflection, this page explains the physical foundation behind wavelength-based optical sensing.
Optical Fiber & Fiber Bragg Grating Basics
1. Physical Structure
Optical fiber consists of three primary layers: core, cladding, and coating.
Light propagates along the fiber core through total internal reflection, while the cladding ensures optical confinement and the coating provides mechanical and environmental protection.
A Fiber Bragg Grating (FBG) is formed by introducing a periodic refractive index modulation directly within the fiber core. It is not an external component, but an intrinsic modification of the fiber itself.
2. Core Mechanism
The reflected wavelength is defined by the Bragg condition:
where:
- neff — effective refractive index
- Λ — grating period
Only wavelengths satisfying this condition are coherently reflected, while the rest of the spectrum is transmitted.
3. System Behavior
A typical FBG contains thousands of periodic structures over a short length (8–10 mm), producing:
- Narrowband reflection
- Stable spectral response
- Insensitivity to optical power fluctuation
4. Engineering Meaning
Physical changes such as strain or temperature alter neff and Λ, resulting in a measurable shift:
This enables conversion from physical variation to wavelength-domain information.
5. Transition
However, the relatively high reflectivity of conventional FBGs introduces limitations in large-scale sensing arrays, particularly due to shadowing and multiple reflection effects.
This is the Bragg condition. The reflected wavelength depends on the effective refractive index n and the grating period Λ. When either parameter changes, the reflected wavelength changes. That is the basis of FBG sensing.
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